Forest fires cannot be prevented... Fire: It’s unfair, the tree wants to take advantage of me

A new study shows that fire has prompted plants to evolve towards high flammability. All of this stems from the "little tricks" of plants to survive.

New tree buds | Tuchong Creative

"Holding fast to the green mountains, its roots are planted in the broken rocks." When the poet used this phrase to praise the bamboo's tenacity, he certainly didn't think of the scene when a wildfire broke out. When a fire sizzles, birds and animals fly away, but plants can only stand there silently and accept their fate, which makes countless people sigh.

We might wonder if plants in fire-prone areas would evolve better fire resistance in order to protect themselves? However, the opposite is true.

A study shows that it was fire that prompted the evolution of plants towards high flammability .

On April 13, 2020, scientists from the United Kingdom and New Zealand published an article in Nature Plants stating that the flammability of plants is closely related to their interspecific affinity and the fire environment in which they live, and that fire may play an important role in the evolution of plants.

Specifically, they came to two conclusions: plants with high affinity tend to have similar flammability, such as eucalyptus, New Zealand tea tree and pine red plum, which all belong to the Myrtaceae family and are representatives of flammable plants. In addition, the flammability of plants is also related to the probability of forest fires in their growth environment. Plants in areas prone to forest fires often do not try to evolve in a fire-resistant direction. Instead, their evolutionary process makes them more and more flammable.

This study confirms a practical experience: many plants can only grow better after being burned, and fire is also an important part of the forest ecosystem. In addition, this study also has important reference value for how to deal with forest fires, especially in the current situation where global warming leads to increasingly frequent forest fires.

Why do plants in fire-prone areas fuel wildfires? What benefits can fire bring to forests? What specific insights does this new research bring us?

Fire is inevitable

Australia wildfires, 2020 | The New York Times

Let's start with the mechanism of forest fire. Simply put, forest fires are inevitable. Apart from the weather/climate factors we know, forests can easily gather all the fire-causing factors - fuel, terrain and ignition. Among them, weather is the most critical, followed by fuel, the amount of which is related to the forest community structure, and terrain is the fuel for the fire. Once all the conditions are met, it is only a matter of time before ignition occurs.

Ignition can be divided into artificial ignition and natural ignition. For forest fires, artificial ignition accounts for a small proportion, because if the conditions for the formation of a forest fire are not met, even intentional ignition (which may be a serious illegal and criminal act) may not necessarily cause a fire.

Natural ignition is different. It has two main causes: one is lightning; the other is that dewdrops can act as convex lenses under sunlight, causing smoldering and then turning into open flames. Because the possibility of fires caused by nature is very high, forest fires are difficult to prevent.

Forest fire is a double-edged sword

Forest fire is burning the ground | Tuchong Creative

Forest fires can indeed cause forest fires, but we don't have to be afraid of fire because fire has a two-sided impact on the forest system.

On the one hand, large-scale catastrophic forest fires may cause serious harm to forests, leading to the destruction of the forest ecological balance for a certain period of time; on the other hand, forest fires of ordinary intensity are an indispensable ecological factor in nature. Fires of a certain frequency and intensity can maintain the balance and stability of the forest ecosystem and keep it healthy.

It can be said that forest fires, like sunlight, water and soil, are important components of the forest ecosystem and play an important role in the forest.

Forest fire affects forest ecosystems in at least the following ways:

1) Forest fire is an important driving force for the succession of forest communities. Fires will cause changes in the forest environment and microclimate. For example, more exposed land allows sunlight to directly hit the surface, increasing the temperature of the soil surface and reducing humidity. Forest fires will also cause the redistribution of other ecological factors, by completely destroying all vegetation, prompting the entire forest to re-succeed (medium and high fires); or by only burning surface vegetation and dead branches and leaves, promoting the degradation of inferior tree species and the development of new tree species, and promoting the succession of forest ecosystems (medium and low surface fires).

2) Forest fires affect the material and energy cycles of the ecosystem. Low-intensity fires can slowly release the energy of the forest ecosystem, promote the transformation of nutrients and species renewal in the forest ecosystem, and are beneficial to the health of the forest ecosystem. After a fire, the forest is more likely to recover.

3) Forest fires promote biodiversity. Small-scale or low-intensity fires make it difficult for flames to reach the tree crowns, which plays a certain "cleaning" role in the forest ecosystem, making it easier for plant seeds and seedlings to contact the soil, promoting the germination of some plant seeds. For example, the cones of Banksia, black spruce, and American yellow pine are baked and cracked by forest fires, and the seeds of dormant Lespedeza seeds are also broken by the high temperature of fire to promote growth. In addition, fire can also maintain the survival of some rare animals, such as North American grouse.

4) Forest fires improve forest ecological resilience. Ecological resilience refers to the ability of an ecosystem to return to a stable state after being disturbed, including the ability to maintain its important characteristics, such as composition, structure, and ecosystem functions. It is a necessary condition for maintaining ecosystem service functions. Climate change, forest fires, and silviculture measures have always affected the resilience of northern forest ecosystems. Studies have shown that for northern temperate forests, a 30% increase in fire disturbance can lead to an increase in forest ecological resilience, while a 200% increase in fire disturbance reduces the mean value of forest ecological resilience.

5) Moderate forest fires can reduce fire risks. For some forest communities, preventing natural fires for a long time can easily lead to larger fires. For example, small surface fires in the forest can clear shrubs and small trees in time. If the shrubs and small trees are allowed to grow, they will gather more forest fuel. Once a fire occurs, the fire will climb up the crown of the big tree along the shrubs and destroy the entire forest. The United States widely uses "planned burning" in forestry to remove accumulated materials in the forest, thereby reducing the possibility of large fires to a certain extent, breaking down large fires into several small fires under artificial control, replacing high-energy fires with low-energy fires, and turning fire damage into fire benefits.

The "little tricks" of plant evolution

A forest of tall, straight eucalyptus trees | Freepik.com

If the above discussion seems a bit like eating from a big pot, the new research allows us to see the "little tricks" of different plants.

The researchers measured the flammability of 194 terrestrial plants and tried to explore the evolutionary pattern of plant flammability from the molecular genetics level. The results showed that closely related plants often have similar flammability, and fire played an important role in the evolution of plant flammability.

There is no doubt that plants do have a motivation to take advantage of fire. In areas prone to forest fires, they increase the probability of forest fires by making themselves more flammable, so that they can enjoy the benefits of forest fires for their own population reproduction and prosperity.

In areas where forest fires are difficult to occur due to climatic and topographical conditions, it is very interesting that plants follow the opposite evolutionary strategy - they become less flammable. This may be because moderate burning does have many benefits, but given the low probability of forest fires in the environment, even if they evolve various flammable traits, they are still difficult to ignite, so they simply don't count on it.

In contrast, in areas prone to forest fires, no matter how hard you try, you can’t escape the fate of being burned once the fire starts. It is better to just burn it thoroughly and catch the “fire” train of population renewal and reproduction.

However, how much burning is appropriate? This also requires a bit of "wisdom". Inadequate burning is like scratching an itch, while over-burning may bring a crisis to the reproduction of the population. After all, the starting point is just to "reshuffle the cards", but it will not end well if it turns into "turning the table".

In fact, many flammable plants have a very accurate grasp of this key issue, which is simply amazing.

Take eucalyptus, the "leader" of Australia's bushfires, for example. They play multiple roles in the bushfires: victims, beneficiaries and even perpetrators. Which role has more screen time depends entirely on the scale and intensity of the bushfires.

Eucalyptus is highly flammable due to the large amount of oil in its bark. When the bark falls off, it accumulates at the roots and spontaneously ignites when the temperature reaches 40 degrees Celsius in summer, causing a fire. Therefore, the eucalyptus tree has been nicknamed "gasoline tree" by the locals.

Eucalyptus leaves unique patterns after peeling | Freepik.com

Under the thick bark of eucalyptus, there are many dormant buds. High temperature can crack the bark and awaken the dormant buds. In addition, forest fires can also crack the shells of eucalyptus seeds, thereby promoting rooting and germination, and taking the lead in the forest ecosystem full of humus after burning.

The most amazing thing is that during the evolution, the trunk of the eucalyptus tree gradually became taller and straighter, and the crown only grew at the top of the tree. When a fire occurs, the hard and thick bark can block the burning of the fire, and the tall trunk can prevent the flames from climbing up, thus preserving the crown. This unique advantage not only makes it not afraid of fire, but also can use forest fires to achieve the purpose of eradicating dissidents and expanding territory. Eucalyptus may even be the "culprit" of forest fires.

Provide important reference for fire prevention and forest protection

Why do scientists want to study this problem? What is its practical significance?

Because most plants that have survived to this day have chosen similar evolutionary strategies under similar fire environments, and closely related plants mostly have similar burning characteristics, the flammability of other plant species can be roughly judged through some known landmark plants.

This also provides data for finding low-flammability species, which can be applied to the construction of biological fire prevention forests and urban planning, helping us better avoid forest fires.

At the same time, since the flammability of plants is closely related to the fire environment, we can also help forest managers predict and prevent regional fires by observing the environments in which highly flammable plants prefer to grow and reproduce.

Complex and sophisticated forest system | Photo provided by the author

Of course, the research on plant flammability is far from over. There may be many unknown factors that affect plant flammability, such as plant growth morphology, phylogeny, and other biological traits. If we make a bold guess, ecologists may be able to estimate the flammability of plants from their external morphology in the future. However, all factors do not exist in isolation, and a more systematic and comprehensive analysis may be needed to accurately predict the flammability of plants.

For now, this research has made us understand that the relationship between forest fires and flammable plants is by no means as simple as "the one who loves you hurts you the most". Plants that have evolved to become more and more "comfortable to burn" are by no means "masochists" but more likely to be "scheming ghosts".

This "palace fight" in nature has been going on for hundreds of millions of years, but the protagonists' will to overcome all difficulties and grow tenaciously has never changed.

Paper information:

Cui XL et al, Shoot flammability of vascular plants is phylogenetically conserved and related to habitat fire-proneness and growth form

https://www.nature.com/articles/s41477-020-0635-1

DOI: 10.1038/s41477-020-0635-1

Author: Zhao Xumao, Young Researcher, Lanzhou University

Editor | Zhang Hao

Editor | Gao Peiwen

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